The present invention generally relates to an electrophotographic apparatus, an electrostatic recording apparatus or the like and more particularly, to a developing device which supplies toner to an electrostatic latent image formed on an electrostatic latent image support member so as to develop the electrostatic latent image into a visible image.
A developing device employing one-component developer has been already proposed in, for example, Japanese Patent Laid-Open Publication No. 143831/1977 in which non-magnetic toner is supplied to a surface of an elastic roller and a blade is pressed against the surface of the elastic roller so as to form an electrically charged toner layer on a outer peripheral surface of the elastic roller such that a toner image is formed by bringing the electrically charged toner layer into direct contact with a surface of the electrostatic latent image support member.
In the prior art developing device, the developing roller is required to be brought into contact with the electrostatic latent image support member uniformly in the axial direction of the electrostatic latent image support member. However, the developing roller or the electrostatic latent image support member itself of the prior art developing device have a slight distortion or dimensional errors which are produced during its manufacture. Thus, the prior art developing device include a drawback in that it is quite difficult to bring the developing roller and the electrostatic latent image support member into contact with each other uniformly in the axial direction of the electrostatic latent image support member.
Meanwhile, in the prior art developing device, the blade is required to be brought into contact with the developing roller at a predetermined pressing contact pressure or more so as to form the electrically charged toner layer on the surface of the developing roller. Therefore, hardness required of the elastic roller is relatively high. On the contrary, a portion of the elastic roller, which is brought into contact with the electrostatic latent image support member, should have a very low hardness in order to prevent damage to the electrostatic latent image and disruption of the image. Thus, the developing roller is required to satisfy entirely opposite hardness requirements. Since such developing roller does not exist, either one of the requirements are neglected undesirably.
Meanwhile, another developing device is known from Japanese Patent Laid-Open Publication No. 77764/1980 in which toner is electrically attached, by using a magnetic brush, to the surface of a roller made of foamed, soft and electrically conductive elastic material and having an electrically conductive film on its surface. In the known developing device, the toner is attached to the electrostatic latent image through contact of the roller with the surface of the electrostatic latent image support member.
However, this prior art developing device has the following disadvantage. Namely, although the roller is made of foamed material, pressing contact pressure of the roller against the electrostatic latent image support member is large. Especially where the peripheral speed of the surface of the roller is different from that of the surface of the electrostatic latent image support member, the image formed on the surface of the electrostatic latent image support member is disrupted.